Ground fault protective systems are intended to sense small differences in current in power lines which normally carry balanced currents. These differences in current may be caused by leakages of current from one of the line conductors to ground, thus depriving the neutral line of some of its normal current which would establish a balance, or zero difference in current, in the lines at the sensor. If the differential currents are below certain predetermined levels, power should normally be allowed to flow uninterrupted. If differential currents should occur which exceed predetermined minima for a long enough time, the circuit should be interrupted, since it is then probable that a malfunction of insulation or perhaps even a serious shock to a human being is occurring.
In practical field use of ground fault interrupters it is often possible for spurious signals to be confused with real fault currents. For example, power line transients due to sudden load changes or to lightninginduced surges can give rise to nuisance tripping in ground fault interrupter systems. Intolerance to frequent nuisance tripping can cause the users of such equipment to establish sensitivity specifications at dangerously high levels. A steady-state spurious signal frequently encountered is a capacitive current to ground from at least one of the downstream power lines. This can be caused by a long cable to the load, or by discrete capacitors such as those used to avoid radio frequency interference, or by similar circuit influences having nothing to do with a true fault on the line. The minimization of nuisance tripping in ground fault interrupter systems by discriminating against reactive currents, spurious transients or other electrical signals having a wave form which does not correspond to or correlate in phase with the line voltage is addressed in my U. S. Pat. No. 3,723,814, issued Mar. 27, 1973 and entitled "Phase-Sensitive Ground Fault Protective Systems."
The term "nuisance tripping" implies, of course, that interruption of the circuit is brought about by the ground fault detector and interrupter system for causes which prove to be insufficient, that the system responds to electrical conditions by needlessly breaking the circuit without the occurrence of a true fault. A true ground fault can have different causes and can give rise to different levels of current imbalance in the supply conductors. If the current imbalance is comparatively high; that is to say, if a comparatively large ground fault current flows, the system should respond quickly and decisively. But if the current imbalance in the supply lines is comparatively small, it is not necessarily desirable for the system to respond as quickly to the signal condition. Under such circumstances the decision to interrupt the supply circuit can be deferred, in a manner of speaking, to permit the system more time to test whether the current imbalance in the supply conductors is a short-term anomaly or a longer term effect representing an actual or incipient ground fault.
In my U. S. Pat. No. 4,024,435 issued on May 17, 1977, "Ground Fault Protective Systems with Variable Rate Integration of Fault Signals" I have demonstrated the importance of integrating ground fault signals at variable rates to minimize the occurrence of nuisance tripping, while perserving the ability of a protective system to respond effectively to ground fault currents as small as 5 milliamperes.
In a typical situation involving possible electrocution of an individual completing a ground fault circuit through his own body, death does not occur instantaneously, but results most often from ventricular fibrillation. The higher the electrocuting current is, the shorter the time in which ventricular fibrillation occurs. The "let-go" current level above which a person experiencing an electrical shock cannot voluntarily release his hold in a physical object through which he received the shock varies according to a number of factors. These variable factors include the weight of the person. Larger people exhibit higher let-go currents. The let-go current for most people is between 10 and 16 ma. But the lower limit of let-go current seems to be 6 ma., at which current level 0.5% of women cannot let go of the object through which they are experiencing electrical shock.
This underscores the importance of preserving the ability of a ground fault protective system to respond to true fault currents as low as 5 milliamperes. Nevertheless it is not at all uncommon for certain electrical systems, particularly those with long power cables connecting the load, to exhibit normal capacitive currents much larger than those which are capable of electrocuting a person. A ground fault protective system sufficiently sensitive to protect human life can be tripped by such differential capacitive currents.